Can't open project file on Google Drive with GanttDroid Pro app

I have a Gantt Project on Google drive. When I try to open it using GanttDroid Pro app on my Android phone I can see the file, but it is grey and I can’t open it.

This is support forum of GanttProject, project management tool for Windows/Linux/Mac OSX desktops.

GanttDroid is a third-party application which is neither developer nor supported by us. Please contact GanttDroid developers.

GanttProject says it is compatible with GanttDroid or visa versa. Thought maybe I need to change a setting on the GanttProject side or something else.

I contacted GanttDroid, but no reply.